The present invention relates to a method and arrangements in a radio receiver system with several standards. More specifically, the invention relates to a method and arrangements of receiving signals in a radio receiver system at the same time with several signal format standards, such as different CDMA, TDMA and FDMA standards. Furthermore, the invention relates to a rake receiver for use in such systems.
Different channel access methods exist for the sending and receiving of digital signals. In TDMA, Time Division Multiple Access, a channel consists of a time slot in a periodic train of time intervals over the same frequency. In FDMA, Frequency Division Multiple Access, a communication channel is a single radio frequency band. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. In contrast, Code Division Multiple Access, CDMA, allows signals to overlap in both time and frequency. Thus, several CDMA signals can share the same frequency band, but the CDMA receiver can also operate at several frequency bands.
In CDMA-technique, simultaneous connections can thus make use of a common frequency band. The selection, i.e. discrimination, between the desired signal and other signals is carried out by suitable signal processing, which is based on that the desired signal is coded. All simultaneous connections have different codes.
In the CDMA system, a radio frequency signal is received by an antenna unit and is downconverted to an intermediate frequency in one or several stages in the analog part of the radio. The intermediate signal is sampled and digitized by an AID converter and filtered in a channelizer to filter out each channel sufficiently well so that signals of other frequencies do not interfere. The result is a bandlimited signal that can be fed to base band processing to reproduce the sent digital data stream by a demodulating unit, which uses for example band spreading technique.
In band spreading, radio signals are transmitted by expanding the bandwidth of the information signal by means of an independent code signal. The most common type of band spreading is direct sequence, in other words direct modulation of the carrier wave with a code or signature sequence, also called pseudo-noise sequence or PN-sequence. The signature sequence usually comprises N bits which are called chips. The entire N-chip sequence is referred to as a transmitted symbol.
In traditional CDMA with direct spreading, the band spreading is achieved so that each fed information bit is replaced by a code sequence that consists of N chips and a spread spectrum is formed.
A plurality of coded information signals modulate a radio freqency carrier and are jointly received as a composite signal at a receiver. If the receiver is authorized and has a synchronous code signal, then the composite signal is correlated with one of the unique codes, and the corresponding information signal can be isolated and demodulated by the actual decoding technique.
There are specifications for the connection between two units with respect to the mechanical connection, the electrical and logic properties of the signals and the rules for the signal sequences. The logic or physical border between two functions is called interface.
In a mobile radio system, the existence of such signal reflecting surfaces as walls, building structures, hills, mountains, etc. are liable to cause the receiver of a radio signal perceiving the signals to be dispersed in time, wherewith signals that carry the same information will arrive at the receiver at different time delays.
To optimally detect the transmitted symbols the rays received must be combined, whereby the signal becomes diversity amplified. Diversity gain can be achieved in advanced signal processing, for example with a rake receiver, wherein several signal components can be used.
A rake-receiver is a radio receiver which utilizes this feature of time dispersed signals. The rake-receiver includes a plurality of independent receiver units, so called rake taps or fingers, each of which receives and tracks, or locates, a respective ray. The rake-receiver also includes means for combining the received signals, and means for delaying these signals so that they will be brought into phase prior to combining the signals. The rake-receiver can usually combine components with separation more than one or equal to one chip-period. The rake fingers or rake taps of a rake receiver in a CDMA-system is adjusted in accordance with the arrival time of the signal.
An ideal sampling is carried out at the strongest place of the signal. If a sample is taken only once during one chip duration, and it is not taken wherein the signal is strongest, then the sampling is not taken optimally in every chip or symbol with non-optimal detection as a result and a systematic error is achieved.
The probability for correct sampling is increased by oversampling, in which several samples are taken for each chip period and the best sample, a decision sample, for each chip can be chosen, which has a start time that indicates the phase of the signal.
The oversampling ratio is defined as the ratio of the signal""s sample rate to the symbol or chip rate. In bandlimiting it means that, the more bandlimited the signal is, the more samples there exist for each chip and thus the oversampling ratio is increased with decreasing bandwidth.
The systematic error connected to the sampling of each chip gets smaller with increased oversampling ratio, but, on the other hand, the oversampling ratio out from the channelizer should be as low as possible so that the receiver system would not need complicated hardware. The lower the oversampling ratio, the farther from the optimum placement one gets, with decreasing sensitivity as a consequence. This sensititvity reduction is called xe2x80x9cdetector lossxe2x80x9d. Thus, the systematic error also depends on the data rate and the equipment performance of the system.
The sampler (i.e., the A/D converters), has a predetermined sample period, which is adjusted in relation to the symbol or chip rate of the radio receiver in a simple way. The adjusted rate is usually a small integer multiple of the symbol rate, f-symbol, or in case of a direct sequence CDMA system, a small integer multiple of the chip rate, f-chip.
With a resampler it is possible to resample from a sample rate that is not an integer times the chip rate to a proper sample rate.
Resampling is usually performed by interpolation between points in the original data sequence, using different techniques. Some of the techniques use extensive computation, but the more bandlimited the signal is (consisting of only relatively low frequencies) the simpler are the satisfactory techniques.
The simplest interpolation techniques are different xe2x80x9choldxe2x80x9d circuits, where first order hold uses the first derivative of the signal to interpolate between points and zero order hold just approximates the desired sample with the closest sample in the original data stream. Zero order hold is of course simpler, but requires a more strongly bandlimited signal in order not to give too high approximation errors. Other resampling techniques often involve higher order filtering or polynomial interpolation.
Resamplers are previously described in for instance U.S. Pat. No. 5,598,439 and U.S. Pat. No. 5,513,209 for demodulation of digitally modulated signals in communication systems. CDMA and rake receivers are decribed in for instance U.S. Pat. No. 5,640,416 and JP-08256084. A previous solution for a multi-rate CDMA communication is presented in EP-814581.
A base station that supports different radio transmission standards is known from EP-815698. The system of this patent can support CDMA and TDMA signals at the same time by using at least two digital channel devices. The channelizer of this patent can only work with one bandwidth and an own channelizer is needed for each standard. A system with several standards having this solution needs quite extensive hardware and the diversity gain is missed.
Using a digital channelizer that can simultaneously support a wide variety of channel bandwidths can minimize the costs for the radio receiver system equipment. Channelizers of this kind are described in the Swedish patent application 9802059-7 of the inventor.
In a conventional one-standard receiver system, the sample rate is chosen in accordance with the chip rate in an integer relation. With several CDMA and other standards with mutually different symbol or chip rates in the system it is not possible to choose the sampling rate so that it is an integer or other simple relation to all chip and symbol rates because of different chip rates in the systems. Therefore a resampler is necessary.
Table 1 shows the various bandwidths and symbol or chip rates for different cellular systems. If several standards would be used simultaneously in the same base station, it is apparent that new technique would be needed to simultaneously handle these different standards in a proper way and without extensive hardware.
An object of the invention is to make such future mobile communication systems and base stations possible, wherein several standards coexist. In such systems, for example, GSM might coexist with wide band CDMA in Europe and in USA D-amps might coexist with both narrow band and wide band IS-95 as well as with GSM.
More in detail, the object of the invention is to make a radio receiver system for such a mobile communication system of the above kind, wherein there is to reduce costs, used common hardware for the handling of the different standards running in the same base station.
Another object of the invention is a base station with several standards, CDMA standards in particular, that have a higher equipment performance than earlier multistandard radio receiver systems.
The method of the invention is mainly characterized in that the demodulating units detects and, when needed, resamples each of the modulated channels individually and delivers corresponding demodulated channels.
The multistandard radio receiver system of the invention is mainly characterized in that the modulated channels are demodulated and, when needed, resampled individually for each standard with a unique signal format.
In the invention, resampling is carried out for the different CDMA and other signal format standards in the base station, to get the proper sample rate for each chip or symbol rate of each standard.
The A/D sampling frequency of the system is chosen in accordance with one of the chip or symbol rates of the different standards in the system. In the invention, resampling is carried out for the other standards.
In the first embodiment of the invention, a proper sample rate for each standard that is in accordance with the chip or symbol rate is achieved in the multi-standards radio receiver system with a separate resampler for each standard. Each standard that needs to have the sample rate resampled in accordance with its chip rate or symbol rate has an own resampler.
Some of the demodulating units in the multi-standards radio receiver might comprise means for resampling and detecting at least one TDMA signal, such units having a TDMA-resampling unit and a TDMA-demodulator/equalizing unit, while other such demodulating unit might comprises means for resampling and detecting at least one CDMA signal, such other units having a CDMA-resampling unit, and a CDMA-demodulator/rake receiver unit. The radio receiver might also comprise other kind of demodulator units and resamplers for those.
The resampler might be before the interface co-located with the channelizer, or alternatively, it can be placed after the interface, co-located with the rake receiver. In the latter case, the data rate over the interface is the same as the out rate of the channelizer, which means that it is lower than in the first case, because the resampler has a high out rate. Concequently, this is a more economic alternative. Between the resampler and the rake receiver, the data rate is high.
One of the systems with a resampler in accordance with the invention, works so that the oversampling rate out from the resampler is much higher than the inrate and thus increases correct sampling.
In addition to the systematic error that might exist in connection with the sampling, the resampling operation gives a certain error, too.
This means that in the two versions of the first embodiment of the invention, there are two error sources in the system, the resampling error and the systematic error, due to the displacement in time, of each rake tap from the optimum position. The total error is the sum of the systematic error and the resampling error.
In the second and preferred embodiment of the invention the resampling and detecting of the modulated channels are carried out in a joint operation in at least one of the demodulating units in the multi-standards radio receiver system. In this embodiment at least one demodulating unit is a CDMA receiver which comprises means for resampling and detecting at least one CDMA signal and has a rake receiver with individual resamplers in each rake finger. An implementation of this type of a rake tap differs from the implementation of a normal rake tap in that it performs joint control of both resampling and demodulation.
The resampling rake taps can use different resampling techniques, which were described earlier in the application. If the simplest zero order hold technique is used, the rake taps keeps track of the timing of the incoming samples and, according to the timing of the chips or symbols in the signature sequence, selects the sample from the incoming signal that is closest to that chip or symbol in time. In interpolation techniques, the desired sample can for instance be interpolated between two incoming samples. In polyphase filtering, the actual phase of the filter is determined by the distance to the closest sample. Different polyphase components are used, which give a given delay to match the incoming time.
In the second embodiment of the invention a low oversampling rate can be used as the rake receiver can receive different sample rates. There is only one error source in the second embodiment of the invention since the average displacement of the rake tap from the optimum position can be kept extremely low. The distance between two signal samples to be correlated in a rake tap will vary, but on an average the number of samples per unit will correspond to the chip rate of the receiver system. Differently stated, the correlation between the code sequence and incoming data stream will in each rake tap be performed on mutually different, non uniformly sampled, sets of data. When sample selection is performed individually on each rake tap, the demodulation, preferably by synchronization with a PN sequence, will be optimal for all rake taps. The only error source will then be the resampling error.
Resampling with a zero order hold in a CDMA system gives at a certain oversampling ratio a certain error. This error is of the same magnitude as the mean displacement error coupled to the sampling of each chip in a rake tap. Other types of resampling will give other error magnitudes.
In accordance with a further embodiment of the invention, the method is extended to higher order hold or other resampling techniques. More extensive computations, such as polynomial interpolation or polyphase filtering, are needed to calculate interpolated sample values according to the resampling technique used. This can be done by resampling or interpolation filter or polynomial interpolator that is synchronized with the PN sequence.
The computational complexity of this resampling or interpolation filter or polynomial interpolator can be much lower in the preferred embodiment of the invention than for a common resampler before the rake taps, since the received signal output need not be oversampled. This compensates for the cost of having individual resampling for each rake tap, at least for systems with only a few channels.
The total computational complexity is probably lower for a system with zero order hold resampling in each rake tap, which therefore is used in the preferred embodiment of the invention.
Since the resampling is done individually in each rake tap, the type of resampling can be individually chosen for each rake tap and also individually optimized. This can be done in conjunction with the estimation of the channel impulse response, so that the best resampling is chosen for the best xe2x80x9crayxe2x80x9d in the multipath channel response, or optimized in some other way that gives a better performance than having all resampling rake taps implemented equal.
The advantages of the inventions are that several standards can be used in the same radio receiver system with less hardware than in previous systems since only one channelizer is needed. A channelizer is often computationally more complex than a resampler. The second embodiment of the invention gives the highest performance and is therefore the preferred embodiment. In this embodiment a more correct sampling is achieved since it is possible to have separate resampling for each rake finger. Furthermore, the method is relatively simple to control and is therefore seen to be very useful.
Existing CDMA receiver systems usually operate with only one signal format standard. In future mobile telecommunications several standards will coexist. The object of the invention is to make such a multi-standards radio receiver system for a mobile communication system of the above kind, wherein there is to reduce costs, used common hardware for the handling of the different standards, CDMA standards in particular, running in the same base station, and which have a higher equipment performance than earlier multistandard receiver systems.
With more than one symbol or chip rate in the system, which is the case when there are simultaneous standards running in the same base station, the chip or symbol rates are not related in such a way to each other that it is possible with existing equipment to choose a sample rate that is an integer or other simple relation of the symbol or chip rate or other simple relation with common hardware for the different standards.
In the invention, resampling is therefore carried out for the different CDMA and other standards in the base station, to get the proper sample rate for each chip or symbol rate of each standard.
The demodulation units of the multi-standards radio receiver system of the invention resamples and detects each of the modulated channels individually and delivers corresponding demodulated channels. In one of the demodulating units, the sampling rate is in accordance with the sampling rate of the A/D converter and resampling is thus not needed.
In the preferred embodiment of the invention, resampling and detecting of the modulated channels are carried out in a joint operation in at least one such demodulating unit in the multi-standards radio receiver system. In this embodiment at least one demodulating unit is a CDMA receiver which comprises means for resampling and detecting at least one CDMA signal and has a rake receiver with individual resamplers in each rake finger.
The invention is also concerned with a rake receiver in accordance with the preferred embodiment of the invention. The rake receiver receives the samples in the time they have and in accordance with one of the resampling techniques, selects the sample from the incoming signal that is closest to that chip in time. As the demodulation and resampling is carried out in a joint operation in a way according to the invention, the error in connection with the displacement of time of the rake taps can be kept low.
In the further embodiments of the invention, the method is extended to individual resampling techniques in different rake taps and to preferred combinations of the oversampling ratio and the resampling technique.
In the following, the invention is explained by means of figures according to some of the embodiments of the invention. The following embodiments are not intended to restrict the invention and many of the details can vary in the scope of the inventive idea. The demodulation can for example be carried out in another way than with syncronization with a PN sequence and the different standards in the radio receiver system can be differently chosen.